Laminates comprising one or more layers of polyimide and one or more layers of metallic substrate material may be used for a variety of applications. For example, polyimide coated metal foils, due to the flexibility and outstanding mechanical, thermal and electrical properties of polyimides, can be used for printed electrical circuits. This is because the laminates are frequently exposed to high temperatures during further processing, for example, during soldering or drilling. The laminates also have to satisfy stringent requirements in regard to their electrical and mechanical properties.
Laminates comprising only one substrate layer of metal or metal alloy and a layer of polyimide, so called single clads, may be used for printed electrical circuits. The same applies to multilayer laminates, so called multi-clads or multilayer circuits, which comprise several metal layers and/or several polyimide layers.
Laminates containing polyimides and metal substrates are well-known in the art. Usually the polyimide layers are bonded to the metal substrate by a conventional adhesive. For example, U.S. Pat. No. 3,900,662, U.S. Pat. No. 3,822,175, and U.S. Pat. No. 3,728,150 disclose bonding of polyimide to metal using an acrylate-based adhesive. However, it has been found that when conventional adhesives such as acrylates, epoxides, polyamides, phenolic resins etc. are used to bond the polyimide to the metal, the resulting laminates do not exhibit entirely satisfactory properties which meet the stringent demands often imposed. Conventional adhesives do not generally possess the high temperature heat stability of the polyimide material itself, and the strength of the adhesive bonds in multilayer laminar polyimide structures deteriorates rapidly when subjected to elevated temperatures.
On account of the disadvantages of laminates comprising layers of conventional adhesives between polyimide and metal, multilayer laminates have been proposed in which the polyimide is bonded directly to metal, i.e., without a layer of adhesive. Thus, British Patent 2,101,526 discloses the bonding of a polyimide derived from biphenyltetracarboxylic dianhydride directly to metal foil by applying heat and pressure. The whole polyimide layer of this laminate, however, is subject to inferior thermal stability as compared to laminates made from layers of conventional polyimides. In addition, the selection of polyimides to be used in such laminates is limited.
U.S. Pat. No. 4,851,495 (Sheppard et al.) discloses polyetherimide oligomers having cross linking and end cap moieties, which provide improved solvent-resistance to cured composites. It also discloses blends generally comprising substantially equimolar amounts of the oligomers and a comparable, compatible, non-cross-linking, etherimide polymer of substantially the same backbone Sheppard utilizes all aromatic moieties with ether (--O --) or thioether (--S--) linkages as flexibilizing functions. To achieve any melt flow away from cure temperatures, m in his formula must be kept no more than 0 or 1. However, this makes the cured resin brittle and suitable only for rigid laminates and/or composites. Even for those applications, brittleness is probably the reason for resorting to blends with reactive plasticizers.
U.S. Pat. No. 4,801,682 (Scola) discloses high temperature polyimides, which are typically the copolymerization product of about 3 mole % to about 42 mole % nadic esters; about 39 mole % to about 49 mole % diamine; and about 17 mole % to about 48 mole % 4,4',9 (2,2,2-trifluoro-1-phenyletheridene)-biphthalic tetracarboxylic acid dialkylester. This chemistry deals with structural composites, where evolution of volatiles is not important. There is an abundance of volatiles because this chemistry involves partial esters of di-and tetracarboxylic acids with lower alcohols, which must be liberated during cure.
U.S. Pat. No. 4,711,964 (Tan et al) discloses bisbenzocyclobutene aromatic imide oligomers. This chemistry also involves structural composites, not suitable for adhesives. Benzocyclobutene end groups may be cured by Diels-Alder conditions requiring high temperatures, and lengthy times, as well as presence of dienenophiles such as commercial bismaleimides, generally leading to brittle resins.
U S. Pat. No. 4,528,373 (D'Alelio et al) discloses unsaturated vinylacetylene-terminated polyimides and processes for their preparation. This invention involves high molecular weight polymers terminated in acetylenic functions requiring high post cure temperatures. The cure temperatures may be lowered by mixing in free radical initiators, which however, are inevitably incorporated in the resin with unknown impact on properties.